The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Referring now to FIG. 1, conventional cable testers 10 are frequently used to isolate cabling problems and to measure cable-lengths. The cable tester 10 is coupled to a cable 14 by a connector 12. The cable 14 can be a multi-conductor cable, and the connector 12 may comprise an RJ-45 or other suitable connector. A connector 15 connects the cable 14 to a load 16. The cable tester 10 typically uses a loop-back module (not shown) as the load 16 at a remote end. The cable tester 10 performs cable analysis—e.g., detects a short, an open, a crossed pair, or a reversed pair—in the cable 14. A short or an open in the cable 14 may be detected without a load. The cable tester 10 can also determine a length of the cable 14 and a distance from one end of the cable 14 to a point where the cable 14 has a fault such as a short or an open condition.
For example, in a multi-conductor cable, a short condition occurs when two or more conductors in the cable 14 are short-circuited together. An open condition occurs when one or more conductors in the cable 14 lack continuity between both ends of the cable 14. A crossed pair occurs when a pair of conductors communicates with different pins at each end of the cable 14. For example, a first pair of conductors may communicate with pins 1 and 2 at one end and pins 3 and 6 at the other end. A reversed pair occurs when two ends in a pair are connected to opposite pins at each end of the cable 14. For example, a conductor connected to pin 1 on one end communicates with pin 2 at the other end, and a conductor connected to pin 2 on one end communicates with pin 1 at the other end.
The cable tester 10 typically employs time domain reflectometry (TDR) (which is based on transmission line theory) to troubleshoot cable faults and to measure cable-lengths. In operation, the cable tester 10 transmits a test pulse 17 on the cable 14 and analyzes a corresponding reflection or a return pulse 18. Specifically, the cable tester 10 measures a difference between a time when the test pulse 17 is transmitted and a time when the return pulse 18 is received. Additionally, the cable tester 10 analyzes characteristics such as shape and size of the return pulse 18 relative to the test pulse 17. Thus, a fault in the cable 14, as well as a length of the cable 14, can be determined based on electrical properties of the cable 14 (e.g., a cable propagation constant) and comparisons between a test pulse 17 and a corresponding return pulse 18.
A conventional cable tester (e.g., cable tester 10), however, may generate inaccurate results when a cable is properly terminated at the remote end. For example, TDR techniques typically cannot be used to determine a length of a cable when the cable is connected (at the remote end) to a link partner that is active or in use. In such a case, the cable functions as a substantially balanced transmission line when the remote end of the cable is properly terminated. With a substantially balanced transmission line, when the remote end receives a TDR pulse (e.g., test pulse 17), the remote end may return a very weak signal. Weak return signals typically cannot be analyzed unless extensive electronic circuits are used. Implementing extensive electronic circuits, however, can be expensive and may not be feasible in low-cost systems.
Alternatively, digital signal processing (DSP) techniques can be used to determine a length of a cable when the cable is connected to an active link. In DSP, unlike in TDR, no pulses are injected into a cable. Instead, parameters such as amplitude, pulse width, pulse shape, etc., of signals that are normally transmitted and received on the cable are measured to determine a length of the cable (or cable-length). DSP, however, involves making some assumptions and therefore yields cable-length measurements that are approximate rather than accurate.
For example, if the length of a cable is determined based on amplitude of a received signal, the amplitude of the transmitted signal is generally unknown or unknowable and, therefore, needs to be assumed. Additionally, any attenuation in the received signal is calculated by assuming an average attenuation per unit length of the cable. Therefore, the length of a cable determined using DSP techniques is generally an approximate estimate rather than an accurate measurement.
Thus, in low-cost systems, since TDR techniques generally cannot be used to analyze reflections from a properly terminated end or an active link, TDR techniques cannot be used to determine a cable-length although the cable may have no faults. Furthermore, although DSP can determine the length of the cable that is properly terminated or that is connected to an active remote end, DSP cannot determine the cable-length accurately when the cable is very long.